my son bought a 1987 ta that runs fine but after cruising for about 10-15 miles at 70-75 the service engine light comes on but it does not put out any codes when checked. does anyone have any idea where to start looking for what would cause this. the car has 168000. miles on it. 305 TPI all original as far as i can see. any help would be greatly app. thanks

 

Check out your egr system.

 

Could be a intermittent misfire code. This will cause the engine light to come on when a few misfires are detected. The light will go out after the car runs for a set amount of time without another misfire or if you cut the car off then on again. I had this problem with mine, turned out to be a dangling plug wire that would ground to the headers. A few zip ties cured that for me.

 

Probably the coolant temp sensor.

Share what you found out when you pulled the codes.

 

when we turned the car off and check for codes all that came up was the 12 code no other codes just the 12 code over and over. go out on the road again and after 10-15 miles the light comes back on

 

Code 12 and nothing else means everything is "normal". I am willing to bet you have an intermittent misfire problem. The only way to know 100% is to use a scan tool to pull the code in real time once it happens. I do not know off the top of my head how many other codes self clear like this.

 

do you need a special type of scanner to read in real time. i just short the pins

 

Jumping the pins require you to turn the key on but not start the engine. If you do not have a scanner you could drive it to your local parts store after the light comes on, leave the engine running so the code does not self clear, and have them use their scan tool to pull the code for you. Some older scanners may instruct you to have key on and engine off. Most of the scanners I have used are able to pull codes and other information while the engine is running.

 

short out B A
[] [] [] [] [x] [x]
[] [] [] [] [] []

first code will blink 1 flash then 2 flashes 4 times this = system OK
any code after is a error CODE

 

Is the coolant temp sensor still the original one, with the round connector with one pin in the middle? If so, try just swapping it out; new ones have a MUCH better connector (which is usually what fails in those, not the sensor itself) that's oval-shaped with 2 terminals. You have to replace the connector aong with the sensor when you do it.

The reason the CTS will cause the light, is that the ECM is smart enough to know that after the engine has been running for a certain length of time, it should reach some certain temp. If it doesn't, it lights the light, because at that point it can recognize that the inputs it's getting from the sensors are BS. When the CTS connector goes bad, it makes the ECM see about a -40° temp with no change, which it knows is impossible; the temp should at least reach 0° in 10 or 15 minutes of running, if not 100° or more. This should give a "low CTS reading" error code which is 15.

Try just swapping it. It's fairly cheeeep and SUPER easy, it's in the top of the thermostat housing right in front of the carb. Has 2 wires, yellow and black. Even if it's not The Problem, the car will probably run better and use less gas if it's replaced, which may well pay for itself in short order.

 

One little problem with your theory. This is OBD1, OBD1 does not detect misfires. Later OBD2 systems can detect a misfire and will throw a code but these old cars do not have that ability.

 

Although its possible it could be a temp sensor its kind of a remote possibility. For starters there are 2 coolant sensor related codes one for reading a temp too low and one for too high. To trip the code it only needs to see a temperature outside of what it deems normal for 3 seconds. So if the ECM reads less than -40 for 3 seconds the code will be set. This means it shouldnt take 15 mins to set that code. On the flip side if the car warms up and the temp sensor reads over 275 for more than 3 seconds it will trip the temp to high code. Although this is possible seeing as how the temp sensor is important for fuel calculations it would probably also cause drive ability issues, especially with it being that far off, which have not been mentioned. It should also store the code. I would say it may not be a bad idea to atleast measure the resistance just to be sure but its not quite so easy on a TPI car. Its not in the thermostat housing like carbed cars its in front of the intake manifold which would be behind the alternator or the AC compressor depending on year cant remember when they swapped places.

 

Code 45 - rich exhaust indicated. Ignition misfires would set this code the way I stated earlier. Most technicians are aware of the fact that any particular OBD1 code could be set by many different malfunctions. You are correct in saying that OBD1 can not DIRECTLY detect ignition misfires. It also can't detect missing or malfunctioning catalytic converter, it is unable to monitor the deterioration of emissions components, and many other limitations that brought along OBD2 standard.

 

Actually a misfire would cause a lean signal. As odd as that may sound o2 sensors read the amount of oxygen in the exhaust gas not fuel. If a cylinder missfires the oxygen will not be converted into other emissions and will be read by the sensor as more 02 (lean). Further more if you had a misfire bad enough to set a trouble code there would definitely be some noticeable drive ability issues beyond a check engine light.